Refrigeration apparatus and method of refrigeration

ABSTRACT

Refrigeration apparatus for cooling or freezing products such as protein materials includes a drum through which heat transfer fluid is circulated. A belt is wrapped around part of the drum surface and the product to be refrigerated is applied to the drum surface, being pressed thereagainst by the belt. Either liquid or triple point carbon dioxide is circulated through the drum to cool the product in contact therewith. Other heat transfer fluids such as D-Limonene or DOWTHERM may also be circulated through the drum. A tank and related apparatus for cooling the heat transfer fluid is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to drum freezers, and in particular tosuch freezers cooled by a liquid cryogen or other heat transfer fluid.

2. Description of the Related Art

There are many advantages to cryogenic freezing of food products whichhave come to light in the past several decades, and in a number ofinstances, carbon dioxide is the cryogen of choice for efficient andeconomical cryogenic freezing applications. Cryogenic carbon dioxidefood freezers often utilize liquid carbon dioxide under pressuresufficient to maintain it in the liquid state and supply it to spraynozzles through which it is injected into the interior of a thermallyinsulated enclosure wherein the food products to be frozen are deliveredto a freezing region, as by being transported on an endless conveyor orthe like. In a CO₂ food freezer, the low temperatures which can beachieved by the creation of solid CO₂, can create a tendency for liquidCO₂ in the lines leading to the spray nozzles to freeze, particularly attimes when there is no flow or only very low flow. Gassing systems havebeen devised and utilized to clear the lines of liquid CO₂ at certaintimes to prevent such freezing.

It is well known in the art to use drum flakers in the chemical processindustry and to a lesser extent, in the food manufacturing industry.Drums have also been used for cooling or freezing various products,including protein materials, as will be discussed herein. Typicalcooling media for drum coolers or freezers are water, chilled water,refrigerated brine solutions, fluorocarbon refrigerants, such aschloro-fluorocarbons or chloro-fluoro-hydrocarbons (CFC's or HCFC's),and mechanically refrigerated ammonia. The use of liquid nitrogen as aheat transfer media for cooling drums has never been successfullycommercialized. Drum freezers are very efficient for cooling orfreezing, (i.e., solidifying) liquid or semi-soft foods or chemicalproducts as compared to other, conventional methods such as tricklechillers, plate or tray freezers, or blast freezer rooms.

For example, a variety of products, such as hamburger meat and otherprotein materials are prepared as a viscous paste and applied to thesurface of a refrigerated drum, which is cooled to a sufficiently lowtemperature so as to cause freezing of the protein material into acontinuous sheet. For example, U.S. Pat. Nos. 4,098,095; 4,337,627; and4,349,575 disclose cooling drums for a freezing viscous paste ofhamburger meat and the like protein materials.

In U.S. Pat. No. 4,098,095, a fluid refrigerant such as FREON or ammoniais circulated through the interior of the drum and the drum is rotated,in synchronism with a belt surrounding a portion of the drum surface. Aviscous paste is applied to a portion of the cooling drum surfacelocated upstream of the belt. The paste material is spread uniformlyacross the drum surface, and the paste material adheres to the drumsurface with freezing occurring upon contact with the drum surface, orshortly thereafter. As the viscous paste travels with the drum surface,interior portions of the paste blanket become frozen and eventually thesurface of the paste blanket remote from the drum surface also freezes.

The paste blanket rotates with the drum to contact the belt. The beltpresses the blanket against the cooling drum surface to aid in theuniform distribution of paste material across the drum surface, therebyproviding a blanket of consistent, uniform thickness. The blanketemerging from the downstream end of the belt is drawn away from thesurface of the freezing drum by a knife blade, and is passed toprocessing equipment located downstream of the refrigeration apparatus.

U.S. Pat. No. 4,337,627 discloses a similar arrangement of a coolingdrum and a conveyor belt surrounding a portion of the drum surface. Thetemperature of the viscous paste and of the cooling drum are maintainedsuch that the viscous paste sticks to the drum surface shortly aftercontact therewith.

U.S. Pat. No. 4,349,575, also by the same inventor, discloses arefrigeration drum with a conveyor belt surrounding a major portion ofthe drum surface. The viscous paste is fed into a nip formed between theupstream end of the conveyor belt and the drum surface, the conveyorbelt pressing the viscous paste into contact with the drum surface andmaintaining such pressure as the paste travels with the rotating drum.The conveyor belt holds the viscous paste in contact with the drumbefore and after freezing of the paste is completed.

European Patent Application No. 332,287 discloses a drum freezer for eggproducts. Liquid nitrogen is circulated in the drum interior whileliquid egg products are deposited, drop by drop on the drum surface.Upon contact with the drum surface, the drops freeze and are collectedfor transport or further processing. A conveyor belt surrounding thedrum surface is not employed.

There is a continuing demand for an improved drum freezer apparatusresulting in increased economy and increased throughput capability.

SUMMARY OF THE INVENTION

It is an object according to the present invention to providerefrigeration apparatus having a drum cooled by liquid and triple pointcarbon dioxide, that is, carbon dioxide where all three phases (solid,liquid and gas) are present.

Another object according to the present invention is to provide arefrigeration apparatus which uses a heat transfer fluid commonly knownas D-Limonene, or, alternatively, DOWTHERM, and to provide improvedmethods and apparatus for the cooling of the D-Limonene heat transferfluid, to prevent air infiltration and to provide other advantages.

These and other objects according the present invention, which willbecome apparent from studying the appended Description and Drawings, areprovided in a refrigeration apparatus for cooling a product comprising:

a drum of heat conducting material, having an outer wall of cylindricalor some other shape with an outer product-contacting surface anddefining an internal cavity;

means for rotatably mounting said drum;

storage means for storing carbon dioxide refrigerant;

means for introducing the carbon dioxide refrigerant into the internalcavity of the drum;

means for contacting the product with the outer drum surface so that thecarbon dioxide refrigerant absorbs heat from the product, causing atleast a portion of the carbon dioxide in the drum cavity to vaporize;

means for removing the product from the outer drum surface;

means for exhausting carbon dioxide vapor from the drum internal cavity;and

means for liquefying the exhausted carbon dioxide vapor and forreturning the carbon dioxide liquid to the storage means.

The carbon dioxide refrigerant can be either in the liquid or possiblythe triple phase state.

Other objects according to the present invention are provided in arefrigeration apparatus for cooling a product, comprising:

a drum of heat conducting material, having an outer cylindrical wallwith an outer product-contacting surface and defining an internalcavity;

means for cooling the drum;

means for rotatably mounting said drum;

flexible webbing surrounding at least a portion of the drum formaintaining the product in contact with the drum outer surface;

means for mounting the webbing to follow movement of the drum surface;and

means for depositing a solid coolant on the flexible webbing to coolproduct surfaces which are out of contact with the drum outer surface.

Further objects according to the present invention are provided inapparatus for cooling a heat transfer fluid, such as D-Limonene,comprising:

a cooling tank for holding the heat transfer fluid, including outletmeans for discharge of the heat transfer fluid when cooled;

injecting means submerged in the heat transfer fluid for injecting aliquid coolant therein to effect turbulent mixing therewith, with heatbeing absorbed from the heat transfer fluid so as to vaporize thecoolant, with said coolant vapor passing out of said heat transferfluid; and

a separator member disposed in said heat transfer fluid between saidinjecting means and said outlet means to prolong the residence time ofthe coolant in the cooling tank to promote vaporization of the coolant,thereby preventing coolant from being carried directly from saidinjecting means to said outlet means.

The injecting means preferably comprise a submerged manifold with spraynozzles spaced therealong to inject a liquid coolant such as carbondioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like elements are referenced alike:

FIG. 1 is a perspective view of a refrigeration apparatus according toprinciples of the present invention;

FIG. 2 is a fragmentary, cross-sectional view taken along the lines 2--2of FIG. 1;

FIG. 3 is a cross-sectional elevational view of a cooling tank fortreating a heat transfer fluid according to principles of the presentinvention;

FIG. 4 is a cross-sectional view of the cooling tank taken along thelines 4--4 of FIG. 3;

FIG. 5 is a schematic diagram of a refrigeration system, using thecooling tank of FIGS. 3 and 4;

FIG. 6 is a perspective view of an alternative embodiment of arefrigerator apparatus according to the principles of the presentinvention; and

FIG. 7 is a fragmentary view of the conveyor belt of FIG. 6, shownpartly broken away.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and initially to FIGS. 1 and 2, arefrigeration apparatus is generally indicated at 10. The refrigerationapparatus comprises a conventional freezing drum 12 of heat conductingmaterial such as metal, including a cylindrical working surface 14 andannular endwalls 16. A coaxial connector 20 is mounted to one endwall16, to provide connection for an inlet line 22 and an outlet line 24.Referring to FIG. 2, the inlet line 22 is coupled through connector 20to a distribution manifold 30 disposed in the interior of freezing drum12, preferably adjacent the longitudinal axis thereof. A plurality ofnozzles 34 are connected at points along manifold 30, and direct heattransfer fluid carried in inlet line 22 toward the inner surface 36 ofdrum 12.

An endless conveyor belt 40 surrounds a portion of the working surface14 of drum 12. The conveyor belt 40 travels about a series of guiderollers 42, and preferably one of the rollers 42 is driven by motor 172(see FIG. 5) so as to propel conveyor belt 40 in the direction of arrows46. The freezing drum 12 is preferably motor-driven for rotation in thedirection of arrows 48, and preferably the speed of rotation of drum 12and the speed of travel of belt 40 are matched.

A feed conveyor 50 delivers a plurality of products 56 to be frozen inthe direction of arrow 52, directing the products toward the opening ofconveyor belt 40. The feed conveyor 50 deposits products 56 onto aloading ramp 60, the products being directed onto belt 40 immediatelyupstream of the nip formed between belt 40 and working surface 14 ofdrum 12. The products are carried by belt 40 into contact with workingsurface 14 as the belt conforms to working surface 14. Belt 40continually presses the products 56 against working surface 14 as theproducts are advanced toward take up conveyor 64. The products aredischarged onto a loading ramp 66, and travel thereacross in thedirection of arrow 68 so as to be deposited on take up belt 64.

The heat transfer fluid in inlet line 22 may comprise any suitablematerial, such as ammonia, which is typically operated at negativepressures. Other conventional materials such as D-Limonene described inU.S. Pat. No. 3,597,355 may also be used, and the D-Limonene may becooled by injecting liquid carbon dioxide, as explained in the patent.However, according to other aspects of the present invention, the heattransfer fluid in the drum may also comprise carbon dioxide operated atlow pressures of about 300 psig. or less, and more preferably at drumpressures ranging between 125 psig. and 60 psig. Most preferably, thecarbon dioxide is maintained at a pressure of about 125 psig. Thesepressure operating ranges have been found to provide very attractiveheat transfer rates without requiring vacuum vessel constructions forthe drum, as is needed for ammonia systems. Also, with the presentinvention, high pressure (i.e., significantly greater than 300 psig.)vessel constructions are avoided.

The carbon dioxide heat transfer medium operated at pressures between 60psig. and 300 psig. preferably comprises a boiling liquid, and maycomprise triple point carbon dioxide, that is, carbon dioxide in whichall three phases (solid, liquid and gas) are in equilibrium.

Referring to FIG. 2, the carbon dioxide is fed through manifold 30 tonozzles 34, impinging on the inner surface 36 of drum 12. The gaseouscomponent of the triple point carbon dioxide fills the upper portion ofdrum 12, while the liquid and any solid components accumulate in a poolat the bottom of drum 12, as indicated in FIG. 2. The drum may also beoperated in a "flooded" or nearly filled condition, as well.

Referring again to FIG. 2, the gaseous component of the carbon dioxideheat transfer medium exits drum 12 through line 24. According to oneaspect of the present invention, the warm carbon dioxide heat transfermedium is mechanically condensed in a condenser stage 70 which is cooledvia an external refrigerant in an external refrigeration loop. Theexternal refrigerant may comprise CFCs or HCFCs for example, which coolsthe carbon dioxide to its described operating point, with cooled carbondioxide exiting condensing unit 70 and returning to drum 12 through feedline 22.

As an alternative, the carbon dioxide can be released from the system asan expendable, or it may be cooled to its triple point or to a liquidstate which enters drum 12 through feed line 22, to provide coolingtherefor. The carbon dioxide vapor exiting drum 12 may be cooled byliquid nitrogen in unit 70, for example, but preferably is cooled to aliquid phase or to a triple point (solid, liquid vapor phase) by amechanical refrigeration system (including a condenser, evaporator, andcompressor, for example) where the carbon dioxide compressor stage iscooled by the evaporator of a second mechanical refrigeration systemusing CFCs or HCFCs or ammonia or some other medium as the heat transferfluid. As shown in FIG. 2, for example, the condenser 70 is cooled bythe evaporator 71 of an external refrigerating unit 73 including a loopwith a second condenser unit in which the warm heat transfer fluid (e.g.CFCs or HCFCs or ammonia) is mechanically compressed.

The drum illustrated in the drawings effectively has an innercylindrical surface formed by the manifold 30. The diameter of manifold30 is much smaller than the diameter of the drum outer surface. Thepresent invention also contemplates drums, or so-called "wheels," inwhich the inner surface of the drum has a diameter only slightly smallerthan the outer surface of the drum. The manifold for the alternativedrum may include radially oriented distribution conduits.

Referring again to FIGS. 1 and 2, the drum belt is treated with carbondioxide snow prior to receiving the product 56. FIG. 1 shows a pair ofconventional snow horns 80 which are fed liquid carbon dioxide in line82, expanding the carbon dioxide to form snow. The rapid expansion ofthe liquid carbon dioxide causes snow being produced in the horns 80 tobe directed toward conveyor belt 40, thus depositing a blanket of snowon the belt surface, immediately upstream of loading ramp 60. Thus, theproducts 56 being loaded onto belt 40 are placed atop a blanket ofcarbon dioxide snow which remains in contact with the products 56 asthey travel about the surface of freezing drum 12. The belt 40 ispressed against the cooling drum, and squeezes out air pockets whichwould otherwise be trapped between the products 56. If desired, the belt40 can be made slightly porous, the packed snow filling the pores toprevent extrusion of the food products therethrough.

Turning now to FIGS. 3-5, an additional aspect of the present inventionwill be described with reference to the use of carbon dioxide to cool aheat transfer fluid commonly known as D-Limonene. This fluid isenvironmentally safe as compared to other solvents used for this purposeand the present invention, in one of its aspects, contemplates the useof D-Limonene as a heat transfer fluid for refrigerating a cooling drum,where the D-Limonene fluid is warmed. A commercially practical coolingsystem using D-Limonene must prevent loss of the D-Limonene whereverpossible, and an efficient arrangement for cooling the D-Limonene mustbe provided.

A cooling tank generally indicated at 90 includes a generallycylindrical body portion 92 and a tapered bottom wall 94. Cooling tanksof other shapes could also be employed. The cooling tank furtherincludes a neck portion 96 connected to the cylindrical body portion 92by an intermediate converging portion 98. The D-Limonene heat transferfluid 100 fills the majority of the cylindrical body portion, and has anupper surface 102. An inlet line 106 for warm D-Limonene heat transferfluid enters the upper portion of cooling tank 90 and is connected to anozzle 108 which directs the heat transfer fluid into region 112 ofcooling tank 90. An inlet line 116 is provided for directing a coolant,preferably liquid carbon dioxide, to a manifold 118. As illustrated inFIG. 3, manifold 118 extends below surface 102 of the D-Limonene heattransfer fluid. The manifold 118 has a lower free end 120 spaced abovethe bottom wall 94 of the tank. A plurality of nozzles 122 aredistributed along manifold 118 and direct liquid carbon dioxide coolantinto the D-Limonene heat transfer fluid, in region 112.

A level indicator 126 is provided for monitoring the level of D-Limoneneheat transfer fluid in cooling tank 90. A barrier wall 128 is located incooling tank 90, extending upwardly from the bottom wall 94 thereof.Preferably, the fluid surface 102 extends slightly below the top ofbarrier wall 128. The bottom wall 94 is preferably tapered, slopingtoward a discharge opening 132. The barrier wall 128 is preferablylocated to one side of the discharge opening and cooperates with thecylindrical wall portion 92 to define the region 112. As can be seen inFIG. 4, the barrier wall 128 has opposed vertical edges 136, 138 spacedfrom the inner surface of cylindrical wall 92. Accordingly, flowchannels 140, 142 are formed on either side of barrier wall 128.

A coolant, preferably liquid carbon dioxide, is injected into theD-Limonene heat transfer fluid to extract heat therefrom. The number,spacing and direction of the nozzles 122 relative to the barrier wall128 and the inner surface 141 of the tank wall are chosen to produce aturbulent mixing of the injected coolant with, the D-Limonene heattransfer fluid. In the preferred embodiment, the liquid carbon dioxideis injected at pressures up to 300 psig. The flow rate of liquid carbondioxide is controlled to obtain a given temperature of D-Limonene heattransfer fluid returning in line 106, so that the liquid carbon dioxideis vaporized, bubbling out of the D-Limonene heat transfer fluid.

It is preferred that the coolant injected into the D-Limonene heattransfer fluid is not entrained in the cooled fluid discharged throughopening 132. The barrier wall 128 according to the present inventionachieves this objective in at least two ways, by confining the mixingarea to insure at least a minimum turbidity, and to lengthen theresidence time of the injected coolant in the D-Limonene heat transferfluid prior to discharge of the fluid from the cooling tank 92. Theregion 112 functions as a mixing chamber of carefully controlledproportions and configuration. The barrier wall 128 also preventsdevelopment of a whirlpool flow pattern in cooling tank 92, furtherinsuring the desired mixing and prolonged residence of the coolant inthe D-Limonene heat transfer fluid.

After mixing in region 112, the D-Limonene/carbon dioxide mixture passesaround the vertical edges of barrier wall 128, through flow channels140, 142 in the direction indicated by the arrows in FIG. 4. A portionof the liquid carbon dioxide is vaporized in region 112 and rises to theupper portion of tank 90. With continued mixing, the liquid carbondioxide entrained in the flow through channels 140, 142 vaporizes as themixture moves toward discharge opening 132, again, with carbon dioxidevapor passing to the upper portions of the cooling tank.

As indicated in FIG. 3, it is generally preferred that the surface 102of the D-Limonene heat transfer fluid be maintained below the upper endof the cylindrical body portion 92. In the preferred embodiment, theD-Limonene heat transfer fluid fills approximately two-thirds of coolingtank 92, leaving approximately one-third of the tank volume forcollection of carbon dioxide vapor.

Under pressure of the evolved carbon dioxide vapor, the upward flowvelocity of the gaseous carbon dioxide increases in the convergingtransition portion 98, with the carbon dioxide vapor entering the neckportion 96 of tank 90. It has been found that particles of D-Limonenefluid may be entrained with the carbon dioxide vapor, and it isdesirable for economic operation of the system that the D-Limonene beretained in cooling tank 92. Accordingly, there is provided a demisterpad 143. The demister pad 143 is preferably of a wire mesh type, whichis available commercially. The D-Limonene heat transfer fluid rejectedby the demister pad 143 accumulates on the wall of neck 96, and flowsdownwardly under gravity across the inner surface of the tank to jointhe fluid present therein.

The carbon dioxide vapor passes through demister pad 143 toward aconventional labyrinthine anti-backflow device. As shown, theanti-backflow device comprises a heavy gas U-trap, but may be replacedby a low pressure check valve if desired, depending, for example, uponthe gas density of the coolant at ambient conditions. The anti-backflowdevice prevents warm, moist ambient air from flowing into cooling tank90 through the neck portion 96. To further aid in eliminating backflow,an inlet line 150 is located in neck portion 96, below demister pad 143.A purging flow of dry gas is fed in line 150 when the cooling tank is"idling", that is, when coolant vapor production falls off. This willoccur, for example, when the temperature of the D-Limonene heat transferfluid is sufficiently low. A steady flow of dry cryogen gas isintroduced at the upper end of cooling tank 90 to preclude ambient airfrom entering the cooling tank. Without the anti-backflow device and thevapor purge system, moisture may condense on the cold surface of theD-Limonene heat transfer fluid, forming water ice which prevents orotherwise hampers pumping of the fluid through the system.

The chilled D-Limonene fluid is pumped to the rotating drum freezer 12.The D-Limonene, under pressure, enters into a fixed distribution headermounted inside the drum freezer, and evolves by pressure from thedistribution header throughout a series of orifices of spray nozzlesarranged to direct the D-Limonene solution to coat the upper portion ofthe drum freezer inner surface. D-Limonene is relatively easy to pump,even at temperatures as low as -140° F. Quite importantly, the directcontact of D-Limonene with liquid carbon dioxide will not cause freezingof the D-Limonene solution, and this represents a significantimprovement over the direct contact refrigeration of other types of heattransfer fluids, which have been known to freeze, thus introducingmaterial handling problems when attempts are made to apply therefrigerated fluid in a useful manner. Optimum heat transfer rates areachieved in part, by a high wetting of the uppermost part of the drumfreezer inner surface with the coldest refrigerated D-Limonene solution.After the D-Limonene is warmed by the products to be cooled or frozen,it falls to the bottom of the drum freezer.

It is preferred that a flow of carbon dioxide vapor is maintained in thedrum to force the warmed D-Limonene solution to the liquid return line.A relatively low pressure, 5 psig., has been found sufficient for thispurpose. The flow of carbon dioxide vapor into the freezer drum alsoprovides the added advantage of preventing moisture buildup on theinterior drum surface as would result from entry of air into the druminterior. In addition to causing a degradation of the D-Limonene fluid,moisture in the drum would reduce the freezing rate.

Referring now to FIG. 5, a schematic diagram of a refrigerating systemincluding the cooling tank 90 and the refrigerating drum 12, is shown.Cooled D-Limonene heat transfer fluid is discharged from tank 90 andcarried by conduit 156 to the inlet line 22 of freezer drum 12.D-Limonene flowing out of freezing drum 12 on line 24 is carried alongconduit 158 to the inlet line 106 of cooling tank 90. A source of liquidcarbon dioxide is schematically indicated at 160 and is connected toinlet 116 of cooling tank 90 through conduit 162. The supply line 82 isalso connected to the liquid carbon dioxide supply 160, to provide afeed for snow horns 80.

A source of gaseous carbon dioxide is schematically indicated at 166 andis connected to the inlet 150 through a conduit 168. The supply ofgaseous carbon dioxide can comprise a pressurized tank, or if sufficientquantities of carbon dioxide vapor are generated in cooling tank 90,they can be retained in a storage vessel for later use.

The refrigerating apparatus 10 schematically illustrated in FIG. 5includes a drive motor 172 which drives a roller 42, thus propellingconveyor belt 40 at a controlled rate of speed. This speed issynchronized with a motor 174 which rotatably drives freezing drum 12,in synchronism with conveyor belt 40.

Referring to FIGS. 3 and 5, a temperature sensor 180 sends a temperaturesignal along conductor 182 to a temperature controller 184. Thetemperature controller 184 is in turn coupled through conductor 186 toan instrument panel 188. The instrument panel 188 controls the flow ofliquid carbon dioxide and gaseous carbon dioxide into cooling tank 90.The instrument panel is coupled through conductor 192 to a control valve194 in conduit 162. Thus, when temperature sensor 180 detects asufficient cooling of the D-Limonene heat transfer fluid, a signal issent by instrument panel 188 to reduce the flow through valve 194. Theinstrument panel 188 preferably includes means for sensing the signal onconductor 192, indicating the rate of flow of liquid carbon dioxide inconduit 162. In response thereto, instrument panel 188 issues a signalon conductor 200 opening control valve 202 when the flow of liquidcarbon dioxide is sufficiently low as to require a purge flow of carbondioxide vapor into the neck of cooling tank 90. Thus, for sufficientlyhigh flow rates of liquid carbon dioxide into cooling tank 90correspondingly large flow rates of carbon dioxide vapor will begenerated by bubbling through the D-Limonene heat transfer fluid 100,thus further insuring that ambient air will not flow back into thecooling tank.

Circuitry means are preferably provided in instrument panel 188 forcorrelating flow rates of liquid carbon dioxide into cooling tank 90with flow rates of carbon dioxide vapor exiting the anti-backflow device144. When the flow of liquid carbon dioxide is throttled back, a needarises for a flow of carbon dioxide purge gas, and an appropriate signalis sent by instrument panel 188 to open the control valve 202, thusproviding a flow of purge gas.

Turning now to FIGS. 6 and 7, an alternative embodiment of arefrigeration apparatus according to the present invention is generallyindicated at 400. The refrigeration apparatus 400 is similar in manyrespects to the apparatus 10 described above, except for the lack ofsnow horns in the preferred embodiment of apparatus 400, and a hollowconveyor belt 402 through which refrigerated coolant, preferablyD-Limonene is circulated. If desired, a covering of carbon dioxide snowor the like can be applied to the surface 404 of conveyor belt 402 incontact with freezing drum 12, although such is not expected to benecessary in light of the continual cooling of belt 402. The conveyorbelt 402 travels about rollers 42 in the direction of arrows 46 with aportion of the belt path extending across the outer surface of freezingdrum 12. Products 60 are received in the nip formed between the outersurface of drum 12 and the surface 404 of conveyor belt 402.

The hollow conveyor belt 402 is formed of two continuous, spaced apartwalls, an outer wall 406 whose outer surface 404 contacts the freezerdrum, and an inner wall 408. Sidewalls 410, 412 span the distancebetween inner and outer walls 408, 406 and seal the ends of the conveyorbelt, thus forming a hollow cavity between walls 408, 406.

As shown in FIG. 7, a coaxial swivel coupling generally indicated at 420is mounted to wall 410 and connects a coaxial line 422 to the hollowinterior chamber of the conveyor belt. The line 422 includes an innercoaxial conduit 424 providing a feed for a refrigerated cooling mediumto fill the hollow chamber of the conveyor belt. The outer coaxial line426 provides a return path, or discharge to allow warmed coolant to bedischarged from the conveyor belt chamber, to external refrigerationapparatus such as that illustrated in FIG. 3.

The swivel coupling 420 is connected through conduit 430 to a bafflewall 432 which divides the conveyor belt interior chamber. Disposed inbaffle wall 432 are a series of outlet nozzles which direct incomingcoolant in the direction of arrow 436. Refrigerated coolant is therebymade to traverse the length of the conveyor belt, returning to theswivel coupling 420 for discharge along conduit 426. As illustrated,coolant flows in a direction opposite to that of the conveyor belttravel, but could be made to flow in the same direction, if desired, byplacing baffle wall 432 on the opposite side of swivel coupling 420 orby other conventional means which are apparent to those skilled in theart. The swivel coupling 420 follows the conveyor belt 402 as it travelsabout its defined path, the swivelling feature preventing entanglementof coaxial conduit 422. As with the aforementioned snow covering on theconveyor belt, the arrangement of apparatus 400 and the hollow conveyorbelt 402 thereof provides cooling for the surfaces of product 56 facingaway from drum 12.

The present invention also contemplates a static charge of refrigeratedfluid coolant within the chamber of conveyor belt 402, although such isgenerally not preferred because of the reduced refrigerating capabilitythat the conveyor belt would offer.

Significant cooling of the D-Limonene heat transfer fluid has beenachieved. For example, the D-Limonene has been cooled to temperaturesranging between -20° to -100° F., in an insulated chamber using expandedliquid carbon dioxide to form solid particles of snow and carbon dioxidevapor in the D-Limonene fluid. The solid particles of carbon dioxidehave been observed to mix thoroughly with the D-Limonene at a highvelocity, and to cool the D-Limonene as the solid particles of carbondioxide absorb heat and undergo a phase change, including sublimation.

With the carbon dioxide-cooled D-Limonene it is possible to provide avery fast freezing rate for food products, sufficient to kill or renderdormant various strains of bacteria and other contaminating organismshave already been shown in a wide variety of food products. Also, asthose skilled in the art will appreciate, it is imperative for varioustypes of food products that a high freezing rate be provided, in orderto avoid various types of degradation of the market value of the foodproducts. The carbon dioxide cooling of the present invention providessuch elevated freezing rates.

Additional advantages are also attained in cooling D-Limonene withcarbon dioxide. It is important to exclude air and water vapor fromentering tank 90, since either would cause contamination and degradationsuch as foaming of the D-Limonene fluid. With the present invention,carbon dioxide vapor is evolved at rates sufficient to prevent theintrusion of air and water.

The drawings and the foregoing descriptions are not intended torepresent the only forms of the invention in regard to the details ofits construction and manner of operation. Changes in form and in theproportion of parts, as well as the substitution of equivalents, arecontemplated as circumstances may suggest or render expedient; andalthough specific terms have been employed, they are intended in ageneric and descriptive sense only and not for the purposes oflimitation, the scope of the invention being delineated by the followingclaims.

What is claimed is:
 1. Refrigeration apparatus for cooling a product,comprising:a drum of heat conducting material, having an outer wall withan inner surface and an outer product-contacting surface and defining aninternal cavity sealed against contact with the product; means forrotatably mounting said drum; storage means for storing carbon dioxiderefrigerant; inlet means for introducing the carbon dioxide refrigerantinto the internal cavity of the drum so as to bring liquid carbondioxide into contact with the inner surface of the drum outer wall;means for contacting the product with the outer drum surface so that theliquid carbon dioxide refrigerant absorbs heat from the product throughthe drum outer wall, causing at least a portion of the liquid carbondioxide in the drum cavity to vaporize; means for removing the productfrom the outer drum surface; outlet means for exhausting carbon dioxidevapor from the drum internal cavity; and means for liquefying theexhausted carbon dioxide vapor and for returning the carbon dioxideliquid to the storage means;
 2. The apparatus of claim 1 wherein thecarbon dioxide refrigerant comprises a pool of liquid in the drumwithout solids being present.
 3. The apparatus of claim 1 wherein thecarbon dioxide refrigerant comprises a pool of liquid and solids in thedrum.
 4. The apparatus of claim 1 wherein the means for liquefying thecarbon dioxide vapor comprises mechanical compressor means.
 5. Theapparatus of claim 1 wherein the drum has a central axis of rotationwhereat said inlet means and said outlet means are located.
 6. Theapparatus of claim 5 wherein said inlet means distributes liquid carbondioxide refrigerant within said internal cavity and said drum isconstructed so that the liquid carbon dioxide refrigerant fallsrelatively unimpeded to lower portion of the drum internal cavity so asto collect thereat in a pool in contact with the drum outer wall.